Conventionally, ballpoint pens such as oil ballpoint pens and gel ink ballpoint pens have been widely used as writing instruments. Such a ballpoint pen generally adopts a structure in which a ballpoint pen tip having a writing ball for writing is rotatably provided at a tip side thereof and an ink container containing ink is connected behind this ballpoint pen tip.
The ballpoint pen tip includes a ball housing formed as a housing space for rotatably housing the writing ball, a back hole extending from the rear end of the ballpoint pen tip to the vicinity of the ball housing, and an ink guide hole allowing the ball housing and the back hole to communicate with each other.
The ink contained in the ink container is supplied to the writing ball in the ball housing through the back hole and the ink guide hole.
Further, a ball receiving seat surface formed as a spherical surface, with which the surface of the writing ball is brought into contact, is provided around the ink guide hole on the inner surface of the ball housing. Moreover, a ball receiving seat portion is provided which receives the writing ball by this ball receiving seat surface. Furthermore, a plurality of channels are provided which extend radially outwardly from the peripheral edge of the ink guide hole to divide the ball receiving seat surface of the ball receiving seat portion, being formed to have a substantially rectangular cross sectional shape with an interval between the inner side surfaces along the extending direction.
To the surface of the writing ball, the ink is not only directly supplied from the ink guide hole, but also through the channels from the ink guide hole. In this way, the ink is simultaneously supplied to a wide range of the surface of the writing ball.
In such a ballpoint pen, a so-called “direct flow” phenomenon in which ink leaks from the pen tip is likely to occur if the ballpoint pen is left unused for a long time. To prevent this phenomenon, in some ballpoint pens, a coil spring for biasing a writing ball toward a tip side is provided in a ballpoint pen tip and a clearance between the tip of a side wall of a ball housing enclosing the writing ball and the surface of the writing ball is narrowed by pressing the writing ball toward the tip side by an elastic force of the coil spring. In this way, a cross-sectional area of an ink supply path for supplying ink to a pen tip is reduced, thereby making it more difficult for the ink to leak from the pen tip and suppressing the occurrence of the direct flow.
Further, if the depth of the channels is so set that the channels do not communicate with the back hole and the ball housing, an excessive ink transfer amount into the ball housing is prevented. This enables the direct flow to be reliably prevented by the coil spring (c.f., e.g., patent document 1).
Here, in the ballpoint pen in which the direct flow is prevented by the coil spring provided in the ballpoint pen tip, ink which is sufficiently supplied toward the pen tip side by shear of the rotating writing ball (for example, ink with a high shear thinning index (n-value)) may be adopted. In this case, if the amount of supply of the ink from the ink guide hole to the ball housing is insufficient, the ink may not be present between the ball and the ball receiving seat surface. Further, this causes problems that it becomes difficult for the ball to rotate and partially faint written traces occur during writing.
On the other hand, ballpoint pen tips such as the following (1) to (3) have been proposed to solve a problem of partially faint written traces.
Moreover, ballpoint pen tips such as (4) have been also proposed for the purpose of smoothly rotating a ball.
(1) A ball receiving seat surface formed as a concave surface corresponding to the spherical surface of a writing ball is provided around an ink guide hole in the bottom surface of a ball housing. The writing ball is received by this ball receiving seat surface. Furthermore, a ballpoint pen tip includes two types of a plurality of channels with different widths, radially extending from the ink guide hole as a center and alternately arranged in a circumferential direction. By increasing the number of these channels, a sufficient ink supply amount toward a pen tip side is assured (c.f., e.g., patent document 2).
(2) A plurality of channels radially extending from an ink guide hole as a center are provided. The cross-sectional area of ends of these channels opposite to the ink guide hole is enlarged. This increases an ink supply amount toward a pen tip side (c.f., e.g., patent document 3).
(3) A plurality of first channels radially extending from an ink guide hole as a center are provided. Moreover, second channels having shorter length and narrower width than these first channels and penetrating through a ball receiving seat portion to allow a ball housing and a back hole to communicate with each other are provided in the bottom surfaces of these first channels. By providing these second channels, flow resistance in an ink supply path for supplying ink to a pen tip is reduced to increase an ink supply amount toward a pen tip side (c.f., e.g., patent document 4).
(4) A plurality of channels radially extending from an ink guide hole as a center are provided. Ink is supplied to a ball receiving seat surface also by communication among these channels, thereby increasing an ink supply amount to the ball receiving seat surface, and the ball receiving seat surface is easily enlarged by this increase in the ink supply amount. In this way, a degree of freedom in setting the area of the ball receiving seat surface with respect to a writing ball is increased to adjust the area of the ball receiving seat surface, whereby defective rotation of the writing ball is prevented to consequently ensure smooth rotation of the writing ball (c.f., e.g., patent document 5).